The proliferation of networked electronic devices continues along with related efforts to increase signaling speed while maintaining signal integrity. The core of a communication link consists of a transmitter that generates the signal, a channel that carries the signal and a receiver that accepts the signal and processes it correctly. Signals can be carried by channels as electrical signals, optical pulses, or electromagnetic signals. For accurate signaling between networked devices or components, the integrity of the signal needs to be maintained. From a physical layer standpoint, this means that if a “1” is sent from the transmitter and down the channel, be it copper, optical fiber or air, the receiver should also determine that the signal is a “1”. This is a fundamental challenge for data transmission system and circuit designers due to the various factors affecting signal integrity.
Signal integrity in data transmission systems is affected by a number of factors. Apart from random noise, atmospheric and man-made noise, there are deterministic factors that contribute to deteriorating signal integrity. Copper-based signal transmission is affected by the limited bandwidth of copper channels and crosstalk from adjacent channels. Optical signals are affected by frequency-independent loss of optical power as they travel down the fiber as well as dispersion-causing mechanisms depending on the type of fiber. Bandwidth limitations in copper and pulse dispersion in optical fibers result in inter-symbol-interference (ISI) at the receiver. Typically, ISI deteriorates the signal as the data rate and the length of the channel increases. ISI is the primary factor limiting transmission distances over copper-based transmission channels and optical fiber at high data-rates (e.g., 10-Gb/sec). Efforts to compensate for ISI and otherwise maintain signal integrity has resulted in various transmitter-side and receiver-side technologies, including equalization.
Equalization is a process of conditioning the electrical signal, either at the transmitter or the receiver to compensate for channel-induced ISI and improve signal integrity. Linear and non-linear equalization techniques have been explored in the literature. One non-linear equalization technique is referred to as decision-feedback equalization (DFE). In DFE, a delay element is introduced into the receiver circuitry and equalization values are combined with an input data stream. To reach optimum DFE performance, the clock sampling edges should occur at the peaks of the combined signals, necessitating a clock recovery circuit (CDR). Also, the first “tap” equalization value should be chosen according to the actual channel response. DFE implementation in a CDR scenario with over-sampled phase detection is challenging due to stringent feedback timing and alignment requirements.